An annoying coworker who thinks that he is an authority on every subject ever conceived. Behavior includes (but is not limited to) butting in to every conversation within ear-shot, complains incessantly, taking over and fucking up other people's projects, picking fights for the sake of fighting, fucking up because he thinks that he knows everything and doesn't need to RTFM or ask questions to more knowledgeable staff members, invites himself along to lunches and happy hours, brags incessantly, physically stinks of rotten cabbage, and crying whenever something doesn't go his way.

Guy 1: "...then we went to the Pizza House for lunch..."
Femus: "You know, the origins of pizza are very interesting. They come from the Aboriginal tribes of Scandinavia and include rack mountable switch plates and overdrive compartments."
Guy 1: "What the fuck are you talking about? Who asked you anyway, you stupid fucking prick."

a guy who is really nice, but he is a femus. the only way to describe him is a femus. a femus is a noun, or an adjective, used to name or describe someone who is just so indescribable the only thing to call them or describe them as is femus.
it is not a good thing to be called a femus!!!